Gonadotropin releasing hormone (GnRH) plays a pivotal role in fertility and reproduction in mammals. It induces the release of luteinising hormone (LH) en follicle stimulating hormone (FSH) from the pituitary. These hormones are responsible for gonadal steroid production and indirectly for gametogenesis. \ud In this thesis, studies are presented which described the development of a highly immunogenic GnRH antigen, as well as the evaluation of this antigen in vaccines in target species. The native GnRH peptide was enlarged to a GnRH-tandem peptide and immunogenicity was further increased by dimerization of the peptide and introduction of foreign amino acids (G6k-GnRH-tandem-dimer). This modified peptide construct was conjugated to ovalbumine. It appeared to be highly effective in immunosterilization, i.e. blocking testosterone production, in male piglets. \ud The role of the individual amino acids of the G6k-GnRH-tandem-dimer on immunosterilization efficacy was established, using alanine replacements. All amino acids, except amino acids at position 2 and 3, could be individually replaced by alanine without affecting efficacy of the vaccine. Peptides with alanine replacements at position 1 or 4 did induce antibodies that were specific for GnRH and did not cross-react with other GnRH-isoforms present in mammals. \ud Immunosterilization by vaccination against GnRH could be a suitable alternative for surgical castration in male piglets. Surgical castration prevents boar taint, but is an animal unfriendly practice, as it is performed without anaesthesia. Immunosterilization may also be beneficial for growth performance. Effects of the time of onset of effective immunosterilization on growth performance were studied. Effective immunosterilization early during the fattening period resulted in a growth performance similar to surgical castrates. In contrast, when immunosterilization was initiated late during the fattening period, growth performance was improved over surgical castrates and carcass quality appeared to be similar to intact male pigs. In addition, immunosterilized pigs of both groups could easily be distinguished from intact male pigs by the size of their testes. \ud Two vaccines with the G6k-GnRH-tandem-dimer antigen in CoVaccine and Carbopol adjuvant were evaluated in sexually mature male ponies. The CoVaccine-treated stallions showed a high antibody response after the second vaccination, subsequently leading to undetectable testosterone levels and affected testis function, while effects in the Carbopol group were only present in one of the four stallions treated. \ud In humane medicine, GnRH vaccination could be an alternative for hormone therapy in prostate cancer patients. Two potential vaccines with the antigen in ISA 51 and in CoVaccine adjuvant were studied in pigs. The latter vaccine appeared to be more effective. It reduced testosterone levels, weights of testes and accessory sex glands in all treated animals. Both vaccines can be considered as non-toxic; no chronic abnormalities were observed in blood, urine, organs or tissues. \ud In conclusion, this vaccine could be a suitable alternative for surgical castration in pigs, eventually leading to a ban on this animal unfriendly practice in the EU. In human medicine, GnRH vaccination could replace hormone therapy in diseases driven by gonadal steroids. In prostate cancer for instance, GnRH vaccination may be a long term effective, cheap and safe alternative
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